* Skip forced arguments in conversion check
This isn't always safe - we have to have also checked the type of the
things we're converting - but in the place where it is safe it can be a
pretty significant saving.
* Use Closures, not NF, in Binders for normal forms
This means we don't need to reduce argument types during unification if
we don't need to. Also, we now try to avoid reducing closures during
unification if they are unified with a metavariable. Together, this
saves a huge amount of unnecessary evaluation in programs that do a lot
of compile time evaluation.
* Didn't mean to update idris2api.ipkg
* Fix dodgy merge
In the `MkFix : f (Fix f) -> Fix f` example, using `Erased` for `f`
makes the type reduce to `[__] (Fix [__]) -> Fix [__]` and because
`[__] e1 ... en` computes to `[__]`, we end up with `[__] -> Fix [__]`
which does not reference `Fix` anymore.
